1. Field of the Invention
The present invention relates to a method of controlling a boiler plant during a switchover period from an air-combustion mode to an oxygen-combustion mode. The invention particularly relates to a method comprising the steps of feeding solid carbonaceous fuel into a furnace of the boiler plant at a rate determined by a fuel feeding scheme, feeding air into the furnace at a rate determined by a descending air feeding scheme, feeding substantially pure oxygen into the furnace at a rate determined by an ascending oxygen feeding scheme, and recirculating flue gas into the furnace at a rate determined by an ascending flue gas recirculating scheme, wherein the fuel feeding scheme, the air feeding scheme and the oxygen feeding scheme are such that the fuel is combusted and flue gas containing residual oxygen is produced.
2. Description of the Related Art
Coal-fired power plants currently account for more than 40% of man-made world-wide carbon dioxide emissions. Oxygen-combustion, also called oxyfuel combustion, is one of the generally known methods of removing carbon dioxide from the exhaust gas of a power plant combusting coal, or other solid carbonaceous fuels. Oxygen-combustion is based on combusting solid carbonaceous fuel with substantially pure oxygen, to produce carbon dioxide and water vapor as the main components of the flue gas. This allows the carbon dioxide to be much more easily captured from the flue gas than in air-combustion, where nitrogen is the dominant flue gas component.
In oxygen-combustion, the fuel is advantageously combusted by using an oxidant consisting of substantially pure oxygen, obtained from an air separation unit (ASU), mixed with recycled flue gas. The products of combustion are then only CO2, water vapor, and a relatively small amount of impurities. The water vapor is condensed in the flue gas channel, yielding a nearly-pure CO2 stream ready for sequestration. The CO2 effluent is then cooled and compressed to high pressure, and the resultant liquid or supercritical CO2 is piped from the plant to be sequestered in geologic formations. The use of an oxidant consisting of oxygen and recycled flue gas renders it possible to adjust the combustion conditions to nearly similar to those of conventional air-combustion boilers.
Due to the uncertainties in, for example, the CO2 capture and storage technology, there is a need for boilers that can be changed from air-combustion to oxygen-combustion, and vice-versa, with minimal changes in the plant equipment. Correspondingly, it is advantageous to be able to operate an oxygen-combustion boiler in an air-combustion mode, for example, when the ASU, CO2 purification and compression unit (CPU), or CO2 storage system is unavailable. Moreover, the flexibility to operate a boiler in either an air-combustion or an oxygen-combustion mode allows adjustment of plant operation to a changing and unknown market for carbon-free electricity, and will promote faster adoption of carbon capture and sequestration technology.
A boiler design that is capable of running on either air-combustion or oxygen-combustion is adaptable to a new boiler or a retrofit of an existing boiler. By properly selecting the flue gas recirculation flow rate, the same boiler geometry, materials, and burners, etc., can be used in both air-combustion and oxygen-combustion modes at all loads. The oxygen-combustion process can readily be retrofitted to an existing boiler by adding the flue gas recycling equipment, the air separation unit, and the CO2 compression and purification equipment.
It would be useful to be able to switch a boiler from air-combustion or oxygen-combustion online, i.e., without interrupting or seriously distracting the combustion process. Especially, when using a process where an oxygen-combustion boiler is started with air-combustion, it is necessary to be able to safely switch online, at full load or partial load, from air-combustion to oxygen-combustion. During the switchover period, the flue gas will be purged to the stack instead of the CPU, until the flue gas reaches a certain CO2 concentration. Thus, in order to reduce O2 consumption and CO2 emission, a fast switchover is desired. A fast switchover, however, tends to adversely affect the boiler performance, such as superheat and reheat steam temperatures. Thus, there is a need for proper dynamic control to assure continuous and safe operation of the plant during a fast switchover from air-combustion or oxygen-combustion.